This invention is in the field of light emitting diodes (LEDs).
The background to the invention may be conveniently summarized in connection with four main subject matters. LEDs, LED bars, LED arrays and Lift-off methods, as follows:
LEDs
LEDs are rectifying semiconductor devices which convert electric energy into non-coherent electromagnetic radiation. The wavelength of the radiation currently extends from the visible to the near infrared, depending upon the bandgap of the semiconductor material used.
Homojunction LEDs operate as follows: For a zero-biased p-n junction in thermal equilibrium, a built-in potential at the junction prevents the majority charge carriers (electrons on the n side and holes on the p side) from diffusing into the opposite side of the junction. Under forward bias, the magnitude of the potential barrier is reduced. As a result, some of the free electrons on the n-side and some of the free holes on the p-side are allowed to diffuse across the junction. Once across, they significantly increase the minority carrier concentrations. The excess carriers then recombine with the majority carrier concentrations. This action tends to return the minority carrier concentrations to their equilibrium values. As a consequence of the recombination of electrons and holes, photons are emitted from within the semiconductor. The energy of the released photons is close in value to that of the energy gap of the semiconductor of which the p-n junction is made. For conversion between photon energy (E) and wavelength (xcex), the following equation applies:       E    ⁡          (              e        ⁢                  xe2x80x83                ⁢        v            )        =            1.2398      λ        ⁢          (              μ        ⁢                  xe2x80x83                ⁢        m            )      
The optical radiation generated by the above process is called electroluminescence. The quantum efficiency xcex7 for a LED is generally defined as the ratio of the number of photons produced to the number of electrons passing through the diode. The internal quantum efficiency xcex7i is evaluated at the p-n junction, whereas the external quantum efficiency xcex7e is evaluated at the exterior of the diode. The external quantum efficiency is always less than the internal quantum efficiency due to optical losses that occur before the photons escape from the emitting surface. Some major causes for the optical losses include internal re-absorption and absorption at the surface. The internal efficiency can exceed 50% and, sometimes, can be close to 100% for devices made of a very high-quality epitaxial material. The external quantum efficiency for a conventional LED is such lower than the internal quantum efficiency, even under optimum conditions.
Most commercial LEDs, both visible and infrared, are fabricated from group III-V compounds. These compounds contain elements such as gallium, indium and aluminum of group III and antimony, arsenic and phosphorus of group V of the periodic table. With the addition of the proper impurities, by diffusion, or grown-in; III-V compounds can be made p- or n-type, to form p-n junctions. They also possess the proper range of band gaps to produce radiation of the required wavelength and efficiency in the conversion of electric energy to radiation. The fabrication of LEDs begins with the preparation of single-crystal substrates usually made of gallium arsenide, about 250-350 xcexcm thick. Both p- and n-type layers are formed over this substrate by depositing layers of semiconductor material from a vapor or from a melt.
The most commonly used LED is the red light-emitting diode, made of gallium arsenide-phosphide on gallium arsenide substrates. An n-type layer is grown over the substrate by vapor-phase deposition followed by a diffusion step to form the p-n function. Ohmic contacts are made by evaporating metallic layers to both n- and p-type materials. The light resulting from optical recombination of electrons and holes is generated near the p-n junction. This light is characterized by a uniform angular distribution; some of this light propagates toward the front surface of the semiconductor diode. Only a small fraction of the light striking the top surface of the diode is at the proper angle of incidence with respect to the surface for transmission beyond the surface due to the large difference in the refractive indices between semiconductor and air. Most of the light is internally reflected and absorbed by the substrate. Hence a typical red LED has only a few percent external quantum efficiency, that is, only a few percent of the electric energy results in external light emission. More efficient and therefore brighter LEDs can be fabricated on a gallium phosphide substrate, which is transparent to the electroluminescent radiation and permits the light to escape upon reflection from the back contact. For brighter LEDs, AlGaAs, with the Al percentage equal to 0-38%, grown on GaAs substrates is used. The AlGAs LEDs are usually about 50 xcexcm thick and are grown on GaAs by liquid-phase epitaxy(LPE). The p-n junctions are diffused. For even brighter LEDs, the AlGAs layers are grown even thicker (xe2x88x92150 xcexcm), and the GaAs substrates are etched off. The thick AlGAs layer becomes the mechanical support. With no substrate and a reflector at the back side one can double the external efficiency.
Visible LEDs are used as solid-state indicator lights and as light sources for numeric and alphanumeric displays. Infrared LEDs are used in optoisolators, remote controls and in optical fiber transmission in order to obtain the highest possible efficiency.
The advantages of LEDs as light sources are their small size, ruggedness, low operating temperature, long life, and compatibility with silicon integrated circuits. They are widely used as status indicators in instruments, cameras, appliances, dashboards, computer terminals, and so forth, and as nighttime illuminators for instrument panels and telephone dials. Visible LEDs are made from III-V compounds. Red, orange, yellow and green LEDs are commercially available. Blue LEDs may be formed of II-VI materials such as ZnSe, or ZnSSe, or from SiC.
LEDs can also be employed to light up a segment of a large numeric display, used for example, on alarm clocks. A small numeric display with seven LEDs can be formed on a single substrate, as commonly used on watches and hand-held calculators. One of the major challenge for LEDs is to make very efficient LEDs, with high external efficiency.
LED BARS
A linear, one-dimensional array of LEDs can be formed from a linear series of sub-arrays, wherein the sub-arrays comprise a semiconductor die with several hundred microscopic LEDs. Each LED is separately addressable and has its own bond pad. Such a die is referred to as an LED bar and the individual LEDs in the array are referred to as xe2x80x9cdotsxe2x80x9d or xe2x80x9cpixelsxe2x80x9d.
LED bars are envisioned as a replacement for lasers in laser-printer applications. In a laser printer, the laser-printer applications. In a laser printer, the laser is scanned across a rotating drum in order to sensitize the drum to the desired pattern, which is then transferred to paper. The use of electronically scanned LED bars for this purpose can result in replacement of the scanning laser with a linear stationary array of microscopic LEDs that are triggered so as to provide the same optical information to the drum, but with fewer moving parts and possibly less expensive electric-optics.
Currently, commercial LED bars are of two types: GaAsP on GaAs substrates and GaAlAs on GaAs. The GaAsP/GaAs bars are grown by Vapor Phase Epitaxy (VPE). Because of the lattice mismatch between GaAsP and GaAs, thick GaAsP layers must be grown of about 50 microns or more thickness and growth time per deposition run is long (5-6 hours). LED bars produced in this fashion are not very efficient and consume much power, and have relatively slow response times.
The second type of LED bar, i.e. GaAlAs/GaAs is grown by Liquid Phase Epitaxy (LPE). LPE growth is cumbersome and does not lead to smooth growth, or thin uniform layers, and is not well suited to the growth of complex structures requiring layers of different III-V compositions.
One of the most important performance requirements for LED bars is dot-to-dot uniformity of the optical output or electroluminescence (or xcex7xcex5). Uniformity of 10 to 15% is currently typical but the marketplace desire xc2x12% or better. Another major requirement is output stability over the lifetime of the LED bar. Currently stability is poor. Another important feature is high brightness, which is presently not very good. Elimination of wire bonding which is currently not available is also highly desirable. Thermal sinking is also important, particularly in the case of inefficient GaAsP bars, in which the brightness is dependent upon operating temperature.
LED ARRAYS
Currently, arrays of LEDs, addressable in two directions (i.e., an X-Y array or X-Y matrix), have been formed of discrete LED chips mounted on printed circuit boards. The resolution of such arrays is limited by the pixel size which is on the order of 200 microns square.
An alternate approach has been to use LED bars to project the light on scanning mirrors. The inclusion of moving parts causes life and speed limitations.
A need exists, therefore, for a monolithic X-Y addressable array with high resolution properties.
LIFT-OFF METHODS
In the fabrication of LEDs, LED bars and LED arrays, it is desirable for a number of reasons, chiefly relating to quantum output efficiency, to utilize thin film epitaxial semiconductor layers for device fabrication. Furthermore, as stated in U.S. Pat. No. 4,883,561 issued Nov. 28, 1989 to Gmitter et al.:
xe2x80x9cIn thin film technology there has always been a tradeoff between the material quality of the film and the ease of depositing that thin film. Epitaxial films represent the highest level of quality, but they must be grown on and area accompanied by cumbersome, expensive, bulk single crystal wafer substrates. For some time, research has focused on the possibility of creating epitaxial quality thin films on arbitrary substrates while maintaining the ultimate in crystalline perfection.
The main approach has been to attempt to reuse the substrate wafer by separating it from the epitaxially grown film; however, to undercut a very thin film over its entire area without adversely affecting the film or the underlying substrate, the selectivity must be extremely high. This is very difficult to achieve. For example, J. C. Fan has described in Journale de Physique, re, Cl, 327 (1982) a process in which an epitaxial film is cleaved away from the substrate on which it is grown. Such cleavage, at heat, is difficult to achieve without damage to the film and/or substrate, or without removal of part of the substrate. Also, in some instances, the cleavage plane ( less than 110 greater than ) and the growth plane ( less than 110 greater than ) of the film may be mutually exclusive.
In a paper by Konagai et al. appearing in J. of Crystal Growth 45, 277-280 (1978) it was shown that a Zn doped p-Gal-xAlxAs layer can be selectively etched from GaAs with HF. This observation was employed in the production of thin film solar cells by the following techniques. In one technique, zinc doped p-Gal-xAlxAs was grown by liquid phase epitaxy (LPE) on a n-GaAs grown layer on a GaAs single crystal substrate. During this LPE growth of the Zn doped Gal-xAlxAs. In diffuses into the surface of the underlying GaAs to form a p-type GaAs layer and hence p-n GaAs junction. The surface p-Gal-xAlxAs is then selectively etched away leaving the p-n junction GaAs layers on the GaAs substrate.
In another solar cell fabrication process Konagai et al describe a xe2x80x9cpeeled film technology,xe2x80x9d which will be referred to here as lift-off technology. A 5 micron thick Ga0.3Al0.7As film is epitaxially grown on a GaAs  less than 111 greater than  substrate by LPE. A 30 micron thick Sn doped n-GaAs layer is then grown over the Ga0.3Al0.7As layer and a p-n junction is formed by diffusing Zn into the specimen utilizing ZnAs2 as the source of Zn. Appropriate electrical contacts are then formed on the films using known photoresist, etch and plating techniques. The surface layer is then covered with a black wax film support layer and the wafer is soaked in an aqueous HF etchant solution. The etchant selectively dissolves the Ga03Al0.7As layer which lies between the thin solar cell p-n junction device layers and the underlying substrate, allowing the solar cell attached to the wax to be lifted off the GaAs substrate for placement on an aluminum substrate. The wax provides support for the lifted off film.
While the technique described above has been described in the literature for over ten years, it was not adopted by the industry. One reason for this was a difficulty encountered in completely undercutting the Ga0.3Al0.7As xe2x80x98releasexe2x80x99 layer in a reasonable time, especially when the area of the film to be lifted-off was large. This difficulty arose due to the formation and entrapment of gas formed as a reaction product of the etching process, within the etched channel. The gas created a bubble in the channel preventing or diminishing further etching and causing cracking in the epitaxial film. The problem could only be partially obviated by using very slow reaction rates (very dilute HF solutions). Since both the time required for lift-off and the risk of damage to the overlying film are important, the process was virtually abandoned.xe2x80x9d
In the Gmitter et al. patent, a lift-off approach was used which comprised selectively etching away a thin release layer positioned between an epitaxial film and the substrate upon which it grows, while causing edges of the epitaxial film to curl upward as the release layer is etched away, thereby providing means for the escape and outdiffusion of the reaction products of the etching process from the area between the film and substrate.
The Gmitter et al. process uses Apiezon (black) wax applied to the front side layer to be separated. The stress in the wax imparts a curvature to the layer being separated or lifted, thereby allowing etching fluid access to the etching front. This process is inherently limited to relatively small areas. The etching front must commence from the outer edge of the total area being lifted off. This result in long lift-off times, for example, up to 24 hours for a 2 cm2 area.
In addition, the curvature necessary for lift-off is caused by a low temperature wax so that no high temperature processing can be done on the backside of the lifted area. This results from the fragile nature of the thin film which must be supported at all times. The film, when supported by the wax on the front side, is curved and cannot be further processed in that shape, without a great deal of difficulty. If the wax is dissolved to allow the film to lay flat, the film must first be transferred to a support by applying the backside surface to a support, in which case, access to the backside is no longer feasible without a further transfer. Presently, samples are cleaved to size, which precludes substrate reuse in full wafer form. Thus, this process is useful only for individual small areas that do not require backside processing. More importantly, there is no known method of registration from one lifted-off area to another. Thus, large scale processing for LED bars and LED arrays using this technique is not presently practical.
The invention is directed to novel LEDs and LED bars and arrays, per se. The present invention is also directed to a new and improved lift-off method and to LEDs, LED bars and LED arrays may be such method.
LIFT-OFF METHODS
In one embodiment of the novel left-off method, a thin release layer is positioned between an epitaxial film and the substrate upon which it is grown. A coating of materials having different coefficients of expansion is applied on the epitaxial film layers. The top structure comprising the coating and the epitaxial layers is then patterned as desired to increase the amount of etchant front by cutting channels to completely laterally separate individual lift-off areas or by cutting slits part way into the epitaxial film.
The entire structure is then brought to a suitable temperature which causes thermal stress between the coating compositions while the structure is subjected to a release etchant resulting in lift-off of individual thin film areas supported by the coating.
Where registration between film areas is desired, such as in the fabrication of LED bars or LED arrays, a coating of material, such as uncured UV epoxy, which is capable of being transformed from a more readily soluble state to a less soluble state by UV radiation is applied over a thin film epilayer formed on a release layer over a substrate. A UV light transparent grid with a plurality of openings is affixed over the transformable coating.
A photomask, with an opaque pattern to cover the openings in the grid, is affixed over the grid. The transformable coating is cured every where except beneath the covered openings by exposing the layer to UV light through the photomask.
The mask is then removed. The uncured portions, i.e., in the openings of the grid are then removed by a solvent down to the epitaxial surface leaving a cured grid layer of epoxy over the thin film surface.
Next, the epitaxial layer is etched away down to the release layer using the openings in the grid to create access for the etchant at the many points across the structure.
The thin film layer may then be lifted off while attached to the support grid of remaining cured transformable material. The backside may then be processed on the wafer (substrate) scale with the wafer registration still retained.
In one of several alternative lift-off embodiments, release and registration is accomplished by forming channels between device areas directly on the thin film and thereby exposing area of the release layer between lift-off areas. The exposed areas are then filled with etchant material. While the exposed areas are so filled, a lift-off support structure, such as UV curable epoxy tape, or other fairly rigid material, is affixed to the frontside of the wafer trapping the etchant in the channels. Eventually, the trapped etchant consumes enough release layer material to enable the lift-off support, together with the underlying lift-off area, to be removed from the underlying substrate with registration intact.
LED AND LED BARS
In accordance with the present invention, thin film epitaxial GaAs/AlGaAs LEDs and LED bars are formed by an Organo-Metallic-Chemical Vapor Deposition (OMCVD) lattice matched process. The p-n junctions are grown during OMCVD of an active GaAs layer which is sandwiched between AlGaAs cladding layers formed on a GaAs or Ga substrate. Preferably, carbon is used for the p-type dopant.
The cladding layers confine injected minority carriers to regions near the p-n junction.
A thin top surface of GaAs (light emitting surface) layer of about 1000 xc3x85, or less, if formed to assist in current spreading at the pixel region. Current spreading is desired at the pixel region to provide uniform current through the p-n junction, but is undesirable beyond the pixel region where it would tend to cause a non-uniform pixel boundary, and for ease in contacting the device. The thin top surface layer also prevents oxidation of the AlGaAs cladding layer.
Various methods are employed to isolate the LED bar dots from each other and to preclude current spreading beyond the desired pixel boundary. One such method is ion or proton bombardment to destroy the crystal quality between dot regions and another is etch isolation through to the p-n junction between pixels. A simple but elegant alternate solution to the problem is to initially grow very thin cladding layers which serve the minority carrier confinement function near the p-n junction region, but are poor lateral conductors due to their thinness and thereby serve to prevent current spreading laterally.
By way of contrast, the currently known art uses a thin cladding layer to spread the current. Also, most LED bars use patterned Zn-diffused junctions to define the pixels. In that case, a thick top layer is used because the Zn diffuses quite deeply. This deep diffusion is useful for current spreading, but may not be easily controllable. In the present invention, which discloses grown epitaxial junctions, ion implantation, etching, or anodization may be used, as aforesaid, to create the high resistance region between pixels. A fourth approach, outlined above, uses a thin highly conductive patterned GaAs layer for current spreading to the pixel boundaries, and a thin and much less laterally conductive AlGaAs cladding layer. The thin GaAs layer (between 500 xc3x85 and 1000 xc3x85, and preferably less than 1000 xc3x85) transmits a large fraction of the light and conducts current to the edges of the pixel, provided the pixel size is not too much larger than 30 xcexcm square. Thus, by patterning the GaAs, the current spreading is limited to the edge of the GaAs contact layer, and the cladding layer does not have to be patterned, leading to better planarity of the surface, and also avoiding the formation of exposed junction edges and associated deleterious perimeter leakage currents.
Optionally, the lift-off methods previously discussed may be employed to separate a front processed LED bar from its substrate, or the back surface of the substrate may simply be metallized to form a back contact for current flow.
LED ARRAYS
In accordance with the invention, LED arrays are formed on a suitable substrate comprising a III-V epitaxial heterojunction, preferably comprising AlGaAs cladding layers with a GaAs carbon doped p-n junction formed between the AlGaAs layers using the OMCVD process described above. Optionally, an etch stop or release layer is formed between the substrate and the epi-layers when it is desired to separate the substrate after front side processing.
A pattern of contact pads and bus bars is then formed on the top (or light emitting) surface. Next, each LED dot, or pixel, is isolated by etching part way through the epi-layers forming isolated dot mesas.
A planar support structure (preferably of light transparent material, such as glass) is then bonded to the top of the mesas by a suitable adhesive, such as light transparent epoxy.
After the support is attached, the substrate is etched off, or cleaved off, leaving the LED film patterned on one side (front side) with contact pads and bus bars attached to the support structure. The remaining side (called the backside) is exposed when the substrate is removed. The backside contacts (running orthogonal to the top side contacts) and bus bars are then formed by photolithography followed by electroplating or evaporation of the metal for the contact to form an LED array of LED pixels addressable in two orthogonal directions.
A multicolor array can be formed by two or more such arrays. In the multicolor embodiment, each array is formed with a different bandgap material to create light emissions of different wavelength and, hence, different colors. The larger bandgap material is formed closer to the top or light emitting surface. The material with the larger bandgap will be transparent to radiation from the smaller bandgap material.
A xe2x80x9csmartxe2x80x9d switch can be formed using an x-y LED array, as described above. The LED array is mounted inside a light transparent pushbutton. The LED X-Y contacts are addressed by a microprocessor, so that a message can be displayed on the face of the button indicating, for example, the button function.
A digital multiplexed infrared (IR) and visible image converter/enhancement system can be formed using the previously described lift-off processes and backside processes to form X-Y arrays of photodetectors and X-Y LED arrays of very thin epi-layers with registered dots and metallization on both sides.
An image, focused on the X-Y detector arrays, is converted to an electrical signal by sequentially detecting the charge or current in each IR detector element. An X-Y photo-detector array, formed as above, is coupled to a microprocessor controlled digital multiplexer comprising an array of transistor gates.
The detected signal is amplified and drives a corresponding visible light emitting dot or pixel in an LED array, resulting in conversion of the IR image to a visible light image. The pixel size can be as small as 25 microns of even less, depending on the wavelength of the light and up to the layer thickness, i.e., approximately 1 micron, resulting in very high resolution and fairly low cost.
The above summary will now be supplemented by a more complete description of the invention in the various embodiments described in connection with the following drawings.